Method and apparatus for manufacturing seamless pipes and tubes



B. BANNISTER Oct. 15, 1935.

METHOD AND APPARATUS FOR MANUFACTURING SEAMLESS PIPES AND TUBES Filed June 2l, 1934 5 Sheets-Sheet 2 B. `BANmsn-ER 2,017,388 METHOD AND APPARATUS FOR MANUFACTURING SEAMLESS PIPES AND TUBES ct. l5, 1935.

Filed June 21, 1934 5 Sheets-Sheet 3 [my: p72' B. BANNISTER 2,017,388 METHOD AND APPARATUS FOR MANUFACTURING SEAMLES PIPES AND TUBES Filed June 2l, 1954 5 Sheets-Sheet 4 B. BANNISTER Oct. 15, l1935.

METHOD AND APPARATUS FOR MANUFACTURING SEAMLESS PIPES AND TUBES 5 Sheets-Sheet 5 Filed June 2l, 1934 PARALLEL 5ECT\ON L [Q1/venan' @RVH/v7* @WMV/STER,

Mgt/M Patented @et 15 'i935 OFFICE Bryant Bannister, Mount Lebanon, Pa..

Application `lune 2l, 1934, Serial No. 731,772

l5 Claims.

This invention relates to an improved method and apparatus for manufacturing seamless pipes and tubes, and more particularly to that operation which is known as piercing, although not 5 limited thereto.

and guiding members, are modified within narrow limits in attempting to acquire a superior workpiece from the mill. It may, therefore, be safely said that, of all rolling operations, seamless tube practice has, in particular, been largely an art. This is for the reason that the metal inthe pass of a seamless pipe or tube mill is moved in a manner which is decidedly complex.

From a consideration of all types of seamless tube operations, it clearly appears that the rst objective was to obtain a work-piece having certain characteristics. The second objective was to alleviate, as much as possible, all conditions not conducive to the free forwardmovement of the metal of the Work-piece.

A severe and injurious upsetting action occurred, and resulted in .inherent defects; often such as could 'not be completely removed by any amount of subsequent processing. 'Ihese defects appeared as annular waves, bulges, inside laps, distortions of the grain structure of the metal, and even tearing was frequently experienced.

In other instances, the sub-surface of the metal disclosed enlarged, or ruptured, grains and tiny cracks. These enlarged grains, and cracks, spread when the work-piece was manipulated, and rendered it entirely unt for service.

This severe upsetting action is due, largely, to the fact that all types of cross-rolling mills tend to feed more metal into the inlet end of the pass than the rolls are capable of moving through the outlet thereof.

It is a well known fact that the profiles of the metal-working rolls which provide the convergent inlet end of the metal pass result in a component opposed to the longitudinal movement of the work-piece. This component is neutralized by a component in the opposite direction, which is provided by the profiles of the rolls which define the divergent outlet end of the metal pass. When the work-piece is in the inlet end solely this component of the rolls effectively retards its rate of longitudinal movement and, as the rotational motion remains unaffected, this retardation is advantageous in that'it assists the centering of the piercing plug.

The neutralizing component in the outlet end of the pass tends to accelerate the longitudinal movement of the work-piece after it leaves the inlet end of the pass. 'I'he rate of longitudinal I movement of the work-piece in the outlet end of the pass is frequently so great as to cause it to be torn to such an extent that it can only be removed from the mill with considerable difficulty.

The combined effects of these resistances to the longitudinal movement of the work-piece results 1 in its leaving the pass with a velocity rarely greater than per cent, and often less than 40 per cent of the velocity the metal-working rolls tend to impart.

Numerous tests of various cross-rolling mills 1 reveal that the cross-sectional area of the metal of the work-piece was first increased a small amount, then rapidly decreased, after which the rate of change was retarded to approximately zero. After this retardation to approximately 2 zero occurred, the area of the metal of the workpiece was again increased. These conditions are obviously not conducive to obtaining a work-piece possessing ideal characteristics but have, in the past, been regarded as a necessary evil.

` If a plastic body is subjected to a strain, deformation begins at the place of greatest shear, and takes place in the direction of greatest shear. The initial deformation of the plastic body should be but slight, and increases subsequently at a rate 3 as rapid as its deformability will permit. In blooming mill practice, by way of example, the bloom is given several preliminary passes in which it is deformed but slightly, before it is rapidly reduced in area, or hogged, as it is known in the 35 art.

This procedure prevents excessive stretching of the metal in any direction, and the elimination of enlarged or ruptured grains, and its accompanying tiny cracks. In all existing types of seamless pipes and tube mills, the metal of the work-piece, moving in the manner described, is excessively upset and viciously distorted, or bogged, with the resulting defects enumerated. Due to this unorderly movement of metal, it has been considered as being next to impossible to obtain an extremely thin-walled work-piece by piercing a solid billet. Therefore, a solid billet is always pierced to obtain a relatively thick-walled work-piece, and the wall of this work-piece subsequently reduced by numerous processing operations if its temperature is maintained sufliciently high by the expedient of reheating. These numerous subsequent operations are costly and render some operations commercially impracticable. 5

In order to obviate the injurious characteristics and defects which are common to all present seamless processes, thin-walled tubing must be produced by maintaining an extremely orderly movement of metal throughout the entire proctube mills.

essing operation. I have discovered that such a movement can be obtained by coordinating the .profiles of the component elements of the pass,

such as the metalworking rolls and a piercing plug, in such manner that the axial advance of the metal is increased in a proper ratio. IThis ratio is, according to my discovery, such as will provide a geometrical progression.

This is a well known geometrical law, and refers to a progression in which each of a series of numbers, after the first, is obtained by multiplying the preceding by a constant multiplier which may be either greater or less than i. As I have applied this law to the art of manufacturing seamless pipes and tubes, it provides for the increasing of the several values of each elementary length increment of the metal of the work-piece in a geometrical progression. That is to say, the width of each of the elementary length increments and the several rates of longitudinal advance of a point in the work-piece, obtaining after successive roll contacts, forma geometrical progression. 'Ihis is` also true of the reduction in the crosssectional area of the metal of the increment when the diameter of the pierced portions of the billet is maintained constant. l

It is one of the objects of the present invention to provide a novely method and apparatus for manufacturing seamless pipes and ltubes at a greatly lowered cost of production, in addition to obtaining a vastly superior article.

Another object is the provision of anovel method and means for consistently obtaining a geometric widening of each of the successive incre- Kments of metal of the Work-piece, or what might be termed a natural iiow thereof, and in this manner reduce the wall thickness in an extremely orderly fashion such as is impossible to perform in using conventional types of seamless pipe and This natural flow results in a great reduction in internal laps and tears, less eccentricity in the work-piece, and the obtaining of leaving and trailing ends having their annular extremities lyingl in single planes at right angles Iwith respect to the axis thereof.. Y

A further object is to provide a novel method and` apparatus for eiecting an accelerated delivery speed in seamless pipe and tube practices, which results in minimizing distortion or ,.crowding at intermediate points in the pass, in addition to extending the life-ineuse of the com- .ponent elements of the mill, such as the metal working rolls, piercing-point and guides, and lowering the amount of power required. 55

A still further object is the provision of a `novel method and apparatusrfor manufacturing seamless pipes and tubes in such manner as to enable 3, theproduction of thinner walls, and the easy processing of alloy steels which have heretofore v been.diilieult,`if not impossible, to work.

These and still further objects will be apparent .after referring to the drawings, in which:

f Figure l is a composite view of a piercing mill constructed in accordance with the teachings of the present invention, and disclosing the relationship of one of the metal working rolls to one of the guide members which is shown, together with a portion of the workf-piece in a plane substantially 90 degrees from its true position.

Figure 2 is a plan of the mill of Figure 1 and 'discloses the manner in which the increments of the metal of the work-piece advance through the pass.

Figure 3 is a plan similar to Figure 42 showing in A:section the relative positions and dimensions of.

aoiaess the increments obtaining after successive roll contacts.

Figure 4 is a plan similar to that of Figure 3, but disclosing a modified piercing plug and its effect on the increments. 5

Figure 5 is a diagram showing the method of ascertaining the taper of the piercing plug of Figure 4. y

Figure 6 is a sectionalelevation of the mill of the invention taken adjacent the outlet end thereof, and with the usual guide members removed for the purpose of simplicity. d

Referring more particularly' to the drawings, the numeral 2 designates a pair of metal working rolls which, when taken together, provide convergent portions 3 of substantial length, and divergent portions 4 of a similar length. The most constricted portions of the pass defined by the metal working rolls 2, which is known in the art as the gorge, is indicated by the letter X. In order to procure a forward feeding effect, the metal working rolls 2 are disposed with their axes of rotation oppositely inclined to each other with respect to a horizontal plane. This inclination is known as the roll bearing angle, and is illustrated in the drawings as being 8 degrees. A tapered piercing plug S is disposed between the metal Working rolls 2, providing a pointed extremity i and a rearward portion 8 which is substantially parallel to the divergent portions 4 thereof. 30 The piercing plug 6 is supported in position by a bar 9. A pair of upper and lower stationary guiding member HJ are intermediately disposed between the metal working rolls 2 and, when taken together, provide opposed contact faces I2 and 3g' centering faces i3. These guiding members iii limit the circumferential growth of the workpiece and divert metal displaced by the metal Working rolls 2 into a longitudinal ow.

According to the teaching of the present inven- 40j tion, the contour of the tapered piercing plug 8 is mathematically related to the adjacent surfaces of the metal working rolls 2 to provide that it will enter the work-piece ill and permit successive in-f crements of metal to be formed and reduced in ments of metal are formed immediately after the piercing plug, or other mandrel, enters the Wcrk. piece; one on either side of the pass. This'principle may be likened to that of the twin-screw, and each of these increments advances helically and is followed by succeeding ones, thus filling the pass.

The natural tendency of the metal displaced by the metal working rolls is to flow circumferentially and thus increase the diameter of the workpiece. The width of each increment, or helically advancing strip, will increase very little without reaction against the guide members i0. The total width being rolled is, of course, made up of increments and spreading can, therefore, take place only at the ends and is always slight.

These successive increments are, by my invention, each advanced in a manner which is based upon the theory of geometrical progressioni', as' previously mentioned. Each term is derived byv multiplying the preceding term by a constant 70 multiplier called the ratio, This principle is applied directly to the width of each succeeding increment of work done by the metal working rolls in advancing the work-piece over the piercing plug. in' this manner the metal of the Work- 75- piece is made to now in a very natural manner, and one such as will distribute the working thereof evenly throughout the metal-working portions of the pass. By adhering to this new teaching, the volume of metal passing any point in the pass is constant. That is to say, as each successive increment is reduced in thickness it is increased in width in the proper proportion, as determined by the ratio" or constant multiplier.

Obviously, the unalterable volume of metal moving past every point in the pass is cross-sectional area times speed. The total volume of metal flowing past all points in the pass being always the same, it follows that either a large area is advancing slowly or a smaller one is advancing more rapidly. The lengths of the increments of metal vary according to the relationship of the metal working rolls with the piercing plug as it tapers toward the outlet end of the pass.

Once having found the common ratio of geometrical progression, the widths of the va rious increments may be calculated and the piercing p lug tapered in direct accordance therewith. lt is, of course, considerably less difficult to change the taper of the piercing plug than the contours of the metal working rolls. However, there are instances wherein a converse methodV is resorted to; as, for instance, when the processing operation is carried out over a cylindrical mandrel, or bar.

The angles of the faces of the metal Working rolls 2 are at l330', and their axes disposed at bearing angles of 8, as previously mentioned.

One method of ascertaining the precise taper of a piercing plug such as will provide for a geometi-ical progression of the metal of the pierced portion of the billet is as follows:

The metal working rolls 2 are graphically illustrated in scala The various diameters of billet size and outside diameter and wall-thickness of the pierced Work-piece are well known, and regulated according to practice. Let it be assumed that it is desired to pierce a billet 6 inches in diameter to a tubular work-piece having an outside diameter of 6% inches, and a wall-thickness of of an inch. These dimensions are graphically represented and leave the dimensions of each successive increment of metal of the workpiece in the metal working portion of the pass to be calculated in geometrical progression. t is desirable to process as many increments as can conveniently be done in order to eect the greatest reduction possible in the thickness of the wall of the work-piece, and to divert this reduction into elongation. The number of increments is therefore nominally selected; and is 8 in the present instance.

The initial reduction effected by the necking down function of the metal-working rolls 2, while drawing the solid billet it into the gorge X, or most constricted portion of the pass, can easily be calculated, or xed from a knowledge of past performance of the metal working rolls illustrated, to be approximately 20 per cent. The area of a circle 6 inches in diameter is 28.3 square inches. Therefore, the area of the section of the billet I4 traveling through the gorge is .80 28.3, or 22.64 square inches. lThis figure is designated as Ag (area at gorge), and its corresponding diameter Dg (diameter at gorge) is found by calculation to be 5% inches.

The theoretical advance of the tubular workpiece i5 from the outlet end of the mill, as defined by the divergent portions of the metal working rolls 2,'per half revolution is .5 1r D. tan

3. effect resulting from the contact of the pierced portion of the work-piece with the guiding members l0, and this hasbeen found from experiment to increase the actual advance to approximately 1.10 times the theoretical. The actual advance of the pierced portion of the billet or tubular work-piece emerging from the outlet end of the mill can, therefore, be'taken as .5 1.l 1r D. tan 8. This advance per half revolution is therefore .5 i.l 3.1416 6.75 .l405 (or 1.64 inches), as this means that the metal in the pass moved 1.64 inches before it left the piercing plug 8. This gure of 1.64 inches is taken to be the width of the last increment in the metal working portion of the pass, and the widths of all preceding increments can be obtained by calculating the geometrical progression. Accordingly, a common ratio of increment width is established and used in proper sequence as each dimension is calculated toward the pointed extremity l of the piercing plug This common ratio may be determined by the use of the following formula:

However, there is an additional advancing v or most constricted portion of the pass, is 22.641.

square inches, as previously mentioned, and all of it must go through the pass, although its speed at any intermediate point varies according to the distance therefrom.

The volume of metal passing any point in the passis, however, constant, since it is the area of the cross-section times speed, and the rate of advance at the gorge can be calculated by simple inverse proportion; therefore,

(or x=.543). The advance of the increments at the outlet end of the pass is then 1.64 inches, and that at the gorge is .543 inch.

The advance of each successive increment at intermediate points of the pass can be obtained by geometrical progression, using the same This same result may be obtained if the various areas had been used: i. e.

I ed@ 722.64 g

par;

and r=1.l'70.

Whichever method is used, it will at once become apparent that since the unchanging volume moving past every point in the pass is equal to area times speed, it follows that in one instance the rate of increase in width of each successive increment as it advances is obtained, and in the other, the rate of decrease in the cross-section moving through the pass. The total volume of metal flowing past all points in the pass is therefore always the same; it being either a large area advancing slowly or a smaller area advancing rapidly,y as previously mentioned.

1 Having ascertained the common ratio (1.17) the widths o! the successive increments may be calculated by rst dividing 1.64-s-1.i7, giving a result of 1.40, the width of the next preceding `5 increment, and' 1.40 divided by 1.17, giving the next, etc., thus working toward the gorge. The widths of each of the eight successive increments are given in the following table, having been obtained by simple calculation: f

Increment a b c d e j a b Increment width 1.64 1.400 1.105 1.02 .872 .745 .636 .543 l5 It is to be noted that in each instance the increment on the opposite side of the pass is desighated by the character (or prime).

Having found the width of each successive increment, the depth thereof must be obtained in order to determine the taper of the piercing plug S.

The unchanging volume owing through the metal working portion of the pass is 1r(d-t)tw;

wherein d is the mean outside diameter of each increment, t the thickness at its mid-portion, and w the width of the increment.

However, due to the fact that the actual advance of the metal through the pass is 1.1 times .the theoreticahthis formula can be utilized approximately as wdtw, as the missing values for any increment can be obtained if the other two are known.

The mean diameter of any increment is easily found, it being as much larger than the "gorge" diameter as the pass is widened by the divergent surfaces 4 of the faces of the rolls 2. 'This widening is twice the product of the distance of the mid-point out from the gorge and the tangent of the roll face angle. That is to say, increment a is of uniform thickness, being in what is known as the parallel zone, and all ci the other increments grow in thickness from this zone toward the gorge X. The average thickness is found from a volume constantwhich is fixed as follows:

fdxtx wx= volume at any point, and since 1r will always cancel out,

this may be used as v dxtxwx.

Therefore, for the outlet, 6.211 times .375 times f From this ligure the mean thickness of all increments may be obtained by dividing 3.83 by the product of mean diameter and the width of any increment. These products appear in the following table, in the last column of which the increment thicknesses are in reality the points oi the plotted taper G of the piercing plug 6, as used to attain an approximate "geometrical progression:

The mean depth of each increment is indicatl ed by the prefix 1, as shown in Figure '3. `Ihe taper of the piercing plug 6 may be plotted along the inward extremities of the lines representing these depths, as shown by the full line G in Fig- 5 ure 3. If this is done, a geometrical progression will be had within close limits. This taper canbe even more closely ascertained by calculating an infinite number of ordinates representing the depth of each increment at numerous points. lo However, this would entail a great amount of needless calculation for all practical purposes. Even if this troublesome method is pursued, the taper obtained would represent a plurality of constantly changing radii, and would be very difil- 15 cult and expensive to produce in the pattern and machine shops.

Referring to Figure 4 of the drawings, the line R represents a modified taper for the piercing plug t. This taper provides a working face which 20 will more readily enter the solid billet il and will also reduce the thrust to which the bar 9 is subjected. In addition, it may be easily ascertained and produced in the pattern and machine shops lwith little diiiiculty. The method of determining 25 this taper is illustrated in Figure 5, and is carried out in the following manner:

'I'he working face of one of the metal working rolls 2 is graphically represented, as shown at 2l. The radius of the largest diameter of the pierc- 30 ing plug 6 is also included, as shown at 6l. A line i 00 is used to graphically represent the center line of the pass. A line 200 is drawn at the roll face angle of 3 30' and made to intersect the line |00 on a line 300. A line 6M is used 35 to dene the usual rearward portion oilV the piercing plug 6 which is parallel to the divergent portions 4 of the metal working'rolls 2. The line R represents a single radius of the' taper desired for the piercing plug 6 which is closest to the 40 plotted geometrical curve G of constantly changing radii. The radius of the pointed extremity I of the piercing plug 6 is taken from the intersection of the lines I 00 and 200 and is indicated at r. The distance from the line 300 to the Ior- 45 ward extremity of the parallel rearward portion of the piercing plug 6, which is indicated at 60|.

is called y, while the distance from the line 2" is designated .'r.

These values are as follows: 50

20=line at 3 30 to the center line I Il of the pass.

60=onehalf of the diameter of the piercing plug 6 (2) 5.653=2.826).

200=line in parallelism with line 20. 55 300=line inch in advance of gorge X. 60o-:line 2 inches long in parallelism with line 20.

=radius of taper (unknown).

r=radius of pointed extremity i of piercing plug so 6 (3A of an inch).

il=a distance oi'/ 6.575.

X==a distance of 2.31. i n f To determine R the following formula is used:

i5 y(xr) R r JfG- and the unknown proves, by calculation, to be approximately 15.3.

The modified taper R permits a substantial geometrical progression of .the successive increments of metal over the piercing plug E.

When using the plotted geometrically progressive taper G the piercing plug 6 provides an arca of metal at the gorge X of the pass of apnroxi- 'l5 dil mately 22.64 square inches, while the area when using the modified taper R provides an area of 25.39 square inches at the same point. This difference amounts to a deviation of only 11.2%, which is rather insignificant so far as practical results are concerned. Obviously these dierences in area between the tapers G and R progressively decrease toward the outlet end of the pass, with a corresponding decrease of percentage of deviation.

As a matter of fact, it will be readily appar-- ent from Figures 3 and d that the last three increments of metal of the work-piece, on both sides of the pass (rt-a b--b, o-c) are processed in true geometric progression even if the modified taper R is used. The term substantial geometrical progression therefore contemplates a reasonable deviation from the theoretical curve, or preferred embodiment, such as within a limit of i8 per cent.

The term tubular work-piece as used in the claims is intended to define one which is hollow before it is completely worked, whether it is introduced to the rolls of the mill in the form of a solid billet or otherwise.

While have shown and described several specific embodiments of my invention, it will be understood that i do not wish tobe iimited exactly thereto, since various modifications may be made without departing from the scope of my invention, as defined by the following claims.

i claim:

l. .apparatus for manufacturing seamless pipes and tubes comprising, in combination, a plurality of metal-working rolls and an intermediateiy disposed mandrel, the cooperating contact surfaces of said rolls and mandrel being constructed. and arranged to provide a pass in which the several values of the elementary length increments of the metal of the work-piece are progressed geometrically.

2. Apparatus for manufacturing szamless pipes and tubes comprising, in combination, a plurality of metal-working rolls and an intermediately disposed mandrel, the cooperating contact surfaces of said rolls and mandrel being constructed and arranged to provide a pass in which the several values of the elementary length increments of the metal of the work-piece are progrossed substantially geometrically.

3. Apparatus for manufacturing seamless pipes and tubes comprising, in combination, a plurality of metal-working rolls and an intermediately disposed mandrel, Isaid mandrel having its contact surface constructed and arranged to provide between it and said metal-working rollsY a pass through which the several values of each of the elementary length increments of the metal of the work-piece are progressed geometrically.

4. Apparatus for manufacturing seamless pipes and tubes comprising, in combination, a plurality of metal-working rolls and an intermediately disposed mandrel, said mandrel having its contact surface constructed and arranged to provide between it and said metal-working rolls a pass through which the several values of each of the elementary length increments of the metal oi the work-piece are progressed substantially geometrically.

5. The method of manufacturing seamless pipes and tubes which includes the steps of helically advancing a tubular work-piece through a pass dened by a plurality of metal-working rolls wherein successive' increments are formed by the successive roll contacts, and working the tubular work-piece in such manner that the mean width of each of its elementary length increments so= formed are in geometrical progression. 6. The method of manufacturing seamless pipes and tubes which includes the steps of helig cally advancing a tubular work-piece through a pass defined by a plurality of metal-working rolls wherein successive increments are formed by the successive roll contacts, and working the tubular work-piece in such manner that the mean width of each of its elementary length increments soa formed are in substantial geometrical progres- '7. The method of manufacturing seamless pipes and tubes which includes the steps of hel is cally advancing a tubular workpiece through a pass defined by a plurality of metal-working rolls wherein successive increments are formed by the successive roll contacts, and working the tubular work-piece in such manner that the mean thicls= ness of each of its elementary length increments so-formed are in geometrical progression.

8. The method of manufacturing seamless pipes and tubes which includes the steps of helln cally advancing a tubular work-piece through a g5 pass defined by a plurality of metal-working rolls wherein successive increments are formed by the successive roll contacts, and working the tubular work-piece in such manner that the mean thickness of each of its elementary length increments go so-formed are in substantial geometrical progress sion.

9. The method of manufacturing seamless pipes and tubes`wliich consists in the steps of helically advancing a tubular work-piece through 35 a pass defined by a plurality of metal-working rolls, wherein successive increments are formed by the successive roll contacts, and changing the several dimensional characteristics of each of the elementary length increments of the tubular no work-piece in geometrical progression.

lo. The method of manufacturing seamless pipes and tubes which consists in the steps of helically advancing a tubular work-piece through a pass defined by a plurality of metal-working 45 rolls, wherein successive increments are formed J by the successive roll contacts, and changing the several dimensional characteristics of each of the elementary length increments of the tubular work-piece in substantial geometrical progresd@ sion;

il. The method of manufacturing seess pipes and tubes which includes the steps of heli cally advancing a, tubular work-piece through a pass defined by a plurality of metal-working rolls o5 wherein successive increments are formed by the successive roll contacts, and working the tubular work-piece in such manner that the mean crosssectional `area of each of its elementary length increments so-formed are in geometrical progression.

12. The method of manufacturing seamless pipes and tubes which includes the steps of helically advancing a tubular work-piece through a pass deined'by a plurality of'metal-working rolls o5 wherein successive increments are formed by the successive roll contacts, and working the tubularl A work-piece in such manner that the mean cil-oss-l sectional area of each of its elementary length increments so-formed are in substantial geomet= t0 rical progression.

13. The method of manufacturing seamless pipes and tubes which includes the steps of helle cally advancing a solid work-piece into a pass deiined by a plurality of metal-working rolls, g5

piercing the solid work-piece, whereby successive increments are formed by the successive roll contacts, the piercing step effecting not more than 30 per cent of the total reduction in cross.- sectional area of the work-piece before it reaches the most constricted portion of the pass, and Working the pierced work-piece in such manner that the several dimensional values of each of its elementary length increments so-formed are in `geometrical progression.

14. The method of manufacturing seamless pipes and tubes which includes the steps of helically advancing a solid Work-piece into a pass dened by a plurality of metal-working rolls, piercing the solid Work-piece', whereby successive increments are formed by the successive roll contacts, the piercing step eecting not more than 30 per cent of the total reduction in crosssectional area of the work-piece before it reaches the most constricted portion of the pass, and. working the pierced work-piece in such manner that the several dimensional values of each of its elementary length increments so-formed are 5 a pass defined by a plurality of metal-working 10 rolls wherein successive increments are formedA by the successive roll contacts, and working the tubular work-piece in such manner that the dimensional characteristics of each of the elementary length increments so-formed are within 15 an 18 per cent deviation of those values which constitute geometrical progression.

VBlfiUiN'I BANNISTEB. 

